It should be helper_login_form. I missed that you gave a URL and had a look, and it does appear that it's out of date -- it's using something that was correct in 1.1 but is incorrect in 1.2.
You need to make sure that your style is updated to support 1.2. If someone updated it for you (or it's a third party style), I'd recommend reporting this to them. Assuming that you don't have any outdated templates any more, I can only assume that the changes were merged but some were missed. You could use the template comparison to see what changes are unexpected to this template, but I would expect that there may be other issues elsewhere.